The invention relates to the signalling of a high-voltage aerial line or more generally an aerial transmission line by a signalling device in the context of air safety. The invention may be used in the field of the prevention of accidents during the day and/or at night of birds owing to impacts with transmission lines.
Without being limited to these materials or these values, standard transmission cables of high-voltage aerial lines most often comprise an aluminium alloy and most often have diameters of from 15 to 32 mm, sometimes up to 44 mm. Those cables are relayed over long distances by pylons which are generally spaced apart by approximately one hundred meters from each other. The voltages used vary according to the countries and needs, for example, between 63 and 90 kV for urban distribution or regional distribution and between 110 and 220 kV for regional exchanges. Values approaching 400 kV and more may be reached in some connections with very high voltage. Modern so-called “ultra-high-voltage” lines have been tested up to at least 1200 kV.
Unlike the low-voltage lines which are relatively close to the ground, at a height of approximately ten meters, high-voltage lines are installed at several tens of meters from the ground, often at a height between 30 and 50 meters from the ground as, for example, in France, but may be positioned at greater heights in accordance with their geographical location and their voltage level, reaching up to approximately 350 m for the tallest pylons. Therefore, those lines are at heights which may interfere not only with low trajectories of aircraft or gliders but also, for example, with the flight of migrating birds or the very rapid descending or ascending movements carried out by hunting raptors. Very-high-voltage lines of 225 or 400 kV and more have problems connected with health impacts, impacts on the landscape, tourism, habitat but also particularly ecological problems connected with collisions and electrocution of large numbers of birds which are regularly seriously injured, or killed, each year if they do not avoid those obstacles. In some regions with a high density of migratory birds, the number of victims may be as high as several thousands of victims/km/year, most often large birds.
This problem is known to electricity transport companies and various associations, in particular associations for the protection of birds. The most effective solution would be to bury the electrical lines but this solution has the disadvantage not only of being difficult but also of having to modify a very large proportion of the existing national and international electrical networks, and may therefore possibly be envisaged only over the very long term. Therefore, different types of signalling systems have been developed and are known in the prior art for signalling the high-voltage lines to aircraft and/or to birds. Each known system addresses similar obstacles but without succeeding in simultaneously overcoming them.
For example, document U.S. Pat. No. 4,885,835 discloses a spherical signalling device which comprises two hollow hemispheres of plastics material which surround a section of an aerial line in the context of signalling intended for aircraft pilots. The hemispheres are screwed together on the ground over approximately half of their perimeter, then the non-screwed portion is moved apart by an operator so as to place the hemispheres around a section of high-voltage line. The hemispheres of plastics material are subsequently screwed over the remainder of their perimeter. The disclosed device further comprises an attachment system using cabling which is intended to be wound around the high-voltage line. This type of system is not particularly well suited to very-high-voltage lines of 400 kV or more, whose cables can become heated to temperatures of up to from 80 to 250° C. That system is also not suitable for visibility at night, winter conditions or foggy conditions.
Other solutions are known in the prior art for signalling aircraft but are still at the experimental stage such as, for example, red-coloured counterweights which are bolted to the cables, or systems of wires twisted around the high-voltage cable. Even if some systems operate on cables of 400 kV and more, the visibility for birds remains ineffective, some systems are far too heavy to be deployed over the whole of a line, or they need tools which are too heavy and impractical for their assembly from a helicopter. The materials used further have problems of ageing, in particular involving the changing of colour, so that those devices are not identifiable by birds, in particular under conditions of fog, snow and/or darkness, and deteriorate over time.
Some solutions are known specifically for signalling intended for birds such as, for example, a helical system in which the devices of helical form and variable diameter are wound around the transmission line. Those helical systems weigh approximately 700 g each and are composed of plastics material, typically a thermoplastic polymer such as PVC, and are red and white in colour. In the prior art, such a helical system comprises two helixes which have a wider diameter than the others and which are visible to birds. The projected surface-area per unit is approximately 16800 mm2 for a total exposed surface of 0.12 m2 when 7 helical devices are arranged on cables between two successive pylons which are spaced apart by approximately 120 m, each device being spaced apart from the following or preceding device by approximately 15 m. This system is currently one of the most deployed on medium-voltage lines (up to 90 kV). Another system which is known from the prior art and which is used, for example, in Sweden, involves suspending a device resembling a bi-coloured card. In both cases, use on high-voltage lines cannot be envisaged owing to the temperatures reached, but in particular owing to the need for insulation products to be set to potential. This is because an insulation material positioned on a high-voltage line generates potentials known as “floating” potentials which result in the production of partial electrical discharges, generally referred to as the “corona effect”, between the insulation components and the air. Those discharges destroy the insulation members, to the extent that they may be totally burnt, without taking into account the drops in voltage following that phenomenon, which may therefore go so far as to bring about a reduction in the voltage at the end of production and therefore potentially a reduction in the transit capacity. Extended use for signalling lines to birds also cannot be envisaged owing to the deterioration, in particular the loss of colour, owing to exposure to UV radiation. Those systems further have disadvantages involving durability owing to embrittlement of the materials used. The systems known in the prior art also have the disadvantage of not being suitable for use at night or during atmospheric influences and are particularly unsuitable for fog, snowy or icy conditions.
Therefore, there is a need to provide a signalling device which can be readily installed on a high-voltage aerial line. Given the dimensions, in particular the height, of the pylons supporting the lines and therefore the lines themselves, the installation of the signalling devices is generally carried out by an operator who is suspended from a helicopter, and each operation for installing a signalling device is therefore difficult and involves risks for the operator.
The signalling devices known from the prior art are distributed over the cables of high-voltage lines between each pylon so as to be visible to birds and/or aircraft pilots. Given the length of each cable, therefore, it is necessary to use a plurality of devices per cable in order to allow the identification of the entire length of a cable between two successive pylons. The weight of the signalling devices is therefore also important in order not to increase the mechanical stresses in the cables and therefore there is also a need to provide signalling devices which are quite light in order to be able to be installed in large numbers along a cable between two successive pylons of an aerial transmission line.
As mentioned above, it is also necessary to provide a device capable of operating equally well on low-voltage, medium-voltage, high-voltage or very-high-voltage lines, or even ultra-high-voltage lines.
In the context of preventing accidents involving birds but also of air safety in regions around airports, it is also necessary to provide a signalling device which is effective both during the day and at night, and for all meteorological conditions, in particular during hazy weather and under conditions of snow and/or frost.